A hydrogenation process conducted in the presence of a catalyst containing manganese on an acid support is described in U.S. Pat. No. 4,987,265. This catalyst is prepared via (a) coprecipitation, at a pH between 7 and 10, of a manganese salt and a salt from which an acid carrier is formed, (b) subsequent calcination at a temperature between 300.degree. C. and 700.degree. C., and (c) reduction with a hydrogen containing gas mixture. A hydrogenation according to U.S. Pat. No. 4,987,265 enables the production of a very high yield of benzaldehyde in the gas phase.
It has long been desired to develop means for converting benzoic acids to their corresponding benzaldehydes in the gas phase at relatively low temperatures, i.e below about 400-450.degree. C. Significant increases in conversion and selectivity rates at such lower temperatures have also been a long sought objective. Heretofore, however, the prior processes have suffered from several drawbacks, including reduced conversion and selectivity when such hydrogenations are conducted in the gas phase below about 400.degree. C. Indeed, at such lower temperatures, prior art processes can typically produce significant, and highly undesired, yields of such by-products as toluene, benzene, and benzyl alcohol.